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In this example a GPR system is modelled consisting two planar bow-tie arranged above a ground plane with a metal pipe target. This configuration has been used previously for experimental and computational studies. Smith and Scott[5] published work carried out on a scale model of a GPR system which was subsequently modelled using the FDTD method by Bourgeois and Smith[6].
The arrangement of the antennas, ground and target are illustrated in Figure 1.
Figure 2 shows the computational geometry of the model.
The work of Bourgeois and Smith showed that modelling could provide good results for GPR systems. Their model, however, dispensed with transmissive boundaries because the amplitude of the reflection from the boundary was too great. As a result they used reflective boundaries and a large domain to enable time windowing of the returned signal. While this approach is valid it is expensive in terms of computational resources, hence the need for a large computer to carry out their work.
In the current model using the Celia code a PML RBC is used. This boundary condition reduces the reflection coefficient substantially over other methods for this type of calculation involving dispersive media. This feature enables the domain boundaries to be located very close to the antennas and target, minimising the volume needed for the calculation and hence reducing the computational memory overhead. The model shown here was computed on a 400MHz PC with 196Mb RAM.
Figure 3 shows contours that illustrate the propagation of the transient pulse within the dielectric, while Figure 4 shows the modelled return signal compared to the measured signal.
Figure 4 0 - Measured Signal Compared to Calculated Signal
In this figure the amplitudes have been scaled as no information was available regarding the amplitude of the measured data.